The present invention is directed to a sewage wastewater treatment system comprising a septic tank and aerobic filter arranged in a manner which maximizes the quality of the effluent permanently discharged from the system.
A septic tank typically provides primary treatment for domestic wastewater where municipal treatment facilities are unavailable. In a conventionally operated septic tank, raw untreated sewage wastewater having a significant concentration of waste solids is introduced into the tank from an adjacent building. In the septic tank, solids separate from the liquid portion of the sewage. Solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear liquid layer between the scum and the sludge. This liquid portion of the wastewater, which exits the discharge end of the tank by means of gravity, a pump, or a siphon, is the septic tank effluent.
The quality of the septic tank effluent primarily determines its subsequent disposition, including the size and kind of any required secondary waste treatment facilities. Such effluent quality is generally measured by the biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen present in the effluent.
Nitrogen in raw untreated wastewater is primarily organic nitrogen combined in proteinaceous material and urea. Decomposition of the organic material by bacteria present in the anaerobic environment of the septic tank changes the organic nitrogen to ammonia nitrogen. Thus, in conventionally treated septic tank effluent, nitrogen is present primarily as ammonia nitrogen.
Secondary treatment of septic tank effluent is typically an aerobic treatment. In addition to its reduction of BOD and TSS, the aerobic environment of secondary treatment causes bacteria to oxidize ammonia nitrogen to nitrate nitrogen, a process known as nitrification. Thus, in a conventional system, nitrogen in the secondary treatment effluent is primarily nitrate nitrogen. The secondary treatment effluent is either discharged directly from the system, or undergoes at least partial recirculation through a recirculation tank as shown in U.S. Pat. No. 5,480,561. However, the nitrogen content of the secondary treatment effluent is often unacceptable.
In order to reduce the nitrogen content of the secondary treatment effluent, the nitrate nitrogen must be converted to a readily removable gaseous form of nitrogen. As disclosed in U.S. Pat. No. 5,531,894, this conversion can be accomplished biologically under anaerobic conditions by denitrifying bacteria. Denitrifying bacteria are capable of converting nitrate to nitrite, followed by production of nitrogen gas (N,) which is released to the atmosphere and thus removed entirely from the effluent. To accomplish such removal, however, the denitrifying bacteria require a source of carbon for cell synthesis. Conventional nitrate-laden secondary treatment effluent does not contain a sufficient source of carbon for the denitrifying bacteria, since the aerobic process which produces the secondary treatment effluent removes carbon sources by reducing the BOD. Nor does a recirculation tank such as that shown in the aforementioned U.S. Pat. No. 5,480,561 supply sufficient carbon. However, as disclosed in U.S. Pat. No. 5,531,894, an actual septic tank which receives raw untreated sewage wastewater does supply sufficient carbon to provide significant nitrogen removal by denitrification.
Nonetheless, the system shown in U.S. Pat. No. 5,531,894 fails to maximize the overall quality of the effluent permanently discharged from the system. This is because the system permanently discharges its effluent directly from a septic tank compartment, which necessarily means that the effluent is a mixture of some denitrified secondarily treated effluent and some primarily treated effluent which has not yet undergone the nitrification or the reduction in BOD and TSS accomplished by secondary treatment in an aerobic filter.
Accordingly, it is one object of the present invention to maximize the quality of septic tank effluent with respect to its BOD, TSS and total nitrogen levels by arranging a septic tank and an aerobic filter in a novel recirculating treatment assembly which provides denitrification but prevents permanent discharge from the treatment assembly of any liquid other than as a filtrate from the aerobic filter.
It is a separate object of the present invention, independent of the previous object, to provide a novel filtrate flow splitter capable of recirculating part of the filtrate through a tank and permanently discharging the remainder while preventing permanent discharge of liquid directly from the tank.
It is a further separate object, independent of the previous objects, to provide denitrification in a meander-type septic tank.